Lawn thatch is often seen as a significant lawn problem causing poor growth and a deterioration in lawn quality.
However, there is a lot of confusion over the extent of the problem, the causes of thatch, and the most effective remedies.
The information below seeks to set the record staight and help you identify if you have a thatch problem and, if so, deal with it.
But first, what exactly is lawn thatch?
Thatch is simply the term used to describe the layer of dead and dying organic matter that accumulates at the base of your lawn on the surface of the soil.
It is composed mainly of the undecomposed stems and roots of the lawn grass itself and most lawns have thatch to some extent.
However a number of common myths seem to have arisen about lawn thatch.
Most lawns have a degree of thatch because many common lawn grasses grow using a system of spreading roots or stems which gradually knit together with their neighbours over time. As the grass continues to grow the older roots and stems die and are replaced by new ones. However, this dead material is rather fibrous and 'woody' and slow to decompose, so it forms a woven mat of material, 'thatch', at the base of your lawn.
A thin layer of thatch, less than half an inch thick, can actually be beneficial. It protects the soil surface, acts as a mulch by preventing soil moisture from evaporating, and increases your lawn's resilience to heavy traffic. Removing this layer completely will harm your lawn.
Problems occur when the layer of thatch gets much thicker than this.
A layer an inch thick actually prevents water from penetrating to the soil. It acts rather like a loofah - think how difficult it is to wet a dry loofah by sprinkling water on to it, the droplets just run off the surface. This deprives your lawn of the moisture, and therefore the essential soil nutrients, needed for healthy growth and increases its susceptibility to disease.
Confusingly a thick layer of thatch can also have the opposite effect in very wet conditions. Think of the loofah again. If you soak it thoroughly it becomes saturated and soggy and slow to dry out. A thick waterlogged thatch layer will suffocate the grass.
New leaves or blades of grass will also need to struggle through the thatch before they can reach the light. In some cases the lawn grasses will simply root into the thatch layer rather than the soil. This leaves them short of nutrients and very vulnerable to lack of moisture in hot, dry spells. Weakened grass also makes your lawn more vulnerable to moss problems.
This is a popular misconception and as a gardener you may be familiar with the advice to rake up all the grass clippings left on your lawn after mowing as they cause thatch.
This is not the case as short clippings have a high moisture content and decompose rapidly. By all means rake them up if you want to keep your lawn looking tidy but not to prevent a thatch problem.
Scarifying, by itself, is not a long term solution to a thatch problem. Vigorously raking your lawn will certainly remove some of the thatch in the short term but it does not address the factors causing the problem in the first place.
For that we will need to look at the main causes of lawn thatch.
Some lawn grasses form thatch more readily than others due to their growth habits. The fine leaved grasses found in high quality ornamental lawns tend to form more thatch than the rye grass typically used in a coarser utility lawn.
This is one of the most common causes of the problem.
We all want our lawns to look green and lush and it is very easy to reach for the high nitrogen fertilizer and turn on the sprinkler system at every opportunity.
The trouble is that overfed and overwatered grass grows at too fast a rate and thatch starts to accumulate faster than it can be broken down.
Combine overfeeding and overwatering with a thatch prone grass variety and you have a potential problem.
Certain soil conditions, such as compacted, waterlogged or very acid soils, also increase the likelihood of thatch problems developing.
Lawn grasses do not grow well in these conditions, nor do such soils support the diverse and thriving population of micro-organisms, fungi and earthworms essential to healthy garden soil.
Without these microorganisms the accumulating thatch layer will be slow to decompose.
To solve your lawn thatch problem you need a two pronged attack followed by preventative measures.
First thin out and remove some of the thatch layer.
You may be able to do this by hand using a spring-tined rake if you have a small lawn and the problem isn't too severe. Otherwise you may have to hire a power rake or vertical mower.
This can be an aggressive process so do it in the cooler conditions of very late summer or early autumn to give your lawn a chance to recover.
If pulling out the thatch has created bald patches you can over seed these with the appropriate grass mix.
Now for the most important part: aerate your lawn by hollow tining.
This process removes cores of soil from the lawn, increasing air and moisture penetration and improving drainage as well as stimulating the activities of microorganisms.
For small lawns you can do this manually, for larger areas it is probably a good idea to hire a powered aerator.
To prevent a return of the problem establish a schedule of maintenance using good lawn care practices.
If you know you are guilty of overfeeding and overwatering, cut down. Follow the instructions when when applying lawn feeds and water only when your lawn actually needs it.
Cut your grass little and often and pay attention to localised areas of compaction around washing lines, children's play areas and other high traffic areas.
If you think you have acid soil, carry out a simple soil pH test and then apply garden lime at the recommended rate. This will also help by increasing microbial activity.
There is more information about other lawn problems at common lawn problems.
Many lawn problems can be prevented by good lawn maintenance practices. Find out more at Lawn Maintenance: How to Keep your Lawn in Great Condition.